IR’s IRS27951/2 are self-oscillating half-bridge drivers especially suitable for LLC resonant DC-DC converter applications. The device provides overcurrent protection, each channel operating frequency up to 500kHz, programmable soft-start time and frequency and dead time, mainly used in LCD and PDP TV, telecom and PC switching power supplies, home audio systems.
The IRS2795(1/2) is a self-oscillating half-bridge driver IC for DC-DC resonant converters, especially for LLC resonant half-bridge converters. Its frequency and dead time can be programmed externally by using two external components. The IC provides overcurrent protection using the on-resistance of the low-side MOSFET. The IC can be stopped by pulling the Voltage below the enable voltage threshold through the external CT/SD pin.
Figure 1 IRS27951/2 block diagram
Figure 2 Typical application circuit diagram of IRS27951/2
IRS27951/2 main features
• Simple primary control solution for fixed and variable frequency DC-DC resonant converters
• Maximum 500kHz output per channel with 50% duty cycle
• The floating voltage of the bootstrap working channel can reach up to 600V
• Programmable minimum and maximum switching frequency
• Programmable soft-start frequency and soft-start time
• Programmable dead time
• Micropower startup and ultra-low quiescent current
• Overcurrent protection with low-side MOSFET Rds(on)
• User-initiated micropower “sleep mode”
• Undervoltage lockout
• Simple design with low component count
• Lead free
• LCD and PDP TV
• Telecom switching power supply, computer switching power supply
• Home audio system
Figure 3 IRS27951 evaluation board circuit diagram
Figure 4 IRS27951 evaluation board PCB layout (top layer)
IRS27951 Evaluation Board
The evaluation board for the IRAC27951SR-240W features the IRS27951 resonant half-bridge controller and the IR11682 dual-channel synchronous rectifier controller. The evaluation board includes a front-end AC-DC rectification stage cascaded with a half-bridge resonant DCDC converter with a 24V output voltage rail. The front end is a conventional rectifier stage with rectifier bridge and EMI filter.
The down converter is a multi-resonant half-bridge LLC converter controlled by the IRS27951 (U1) controller HVIC. The controller drives two half-bridge MOSFETs with dead time and a fixed 50% duty cycle, varying the frequency based on a feedback signal to regulate the output voltage for changes in load and input voltage. As mentioned earlier, in addition to current protection, all key functions required to control the resonant converter design can be programmed externally using its 8-pin controller IC.
In this reference design, the IRS27951 is self-powered. The start-up resistors Rstart1~Rstart3 provide start-up current for the IRS27951 during power-on and charging the VCC capacitors (CDC2 and CVcc1). Once the Vcc voltage exceeds the Vccuv+ threshold, the IRS27951 starts to operate and the auxiliary winding of the power transformer provides bias to the IC. When the 24V load changes from 0A~10A, the voltage of the auxiliary winding may vary greatly, so a linear regulator (including DZ4, Rvcc and Q2) is used to keep VCC at 14.5V.
The transformer adopts the magnetic integration method, which integrates the resonant series and the parallel inductance on the transformer. The configuration choice for the transformer secondary winding is center-tapped. The feedback loop is a classic configuration, using the TL431 (U3) to adjust the current of the optocoupler TLP621 (U2). The optocoupler transistor adjusts the switching frequency by adjusting the current of RT on the pin of the controller IC, thereby realizing the regulation of the output voltage.
The secondary rectification is realized by the synchronous rectification controller IR11682 and two PQFN power MOSFETs. Each output pin uses an IRFH5006 and a 60V MOSFET (on-resistance typical 3.5mohm). Using synchronous rectification greatly reduces conducted power losses. The 10A continuous output current does not require a heatsink. Using the highly integrated dual-channel SmartRectifierTM controller IR11682 also saves PCB area.
A synchronous rectification circuit is connected in a low-side configuration. Therefore, the IR11682 can directly drive two SR MOSFETs. An RCD circuit is added to the sense input of the IR11682 VD to provide a leading edge filter and turn-off delay compensation.
The R11682 is biased to 24V by a simple linear regulator Q1 (generally NPN transistor) and Zener diode Dz2 (12V). In addition, the second Zener diode Dz3 (9.1V) is used to prevent the synchronous rectification circuit from being activated when the output voltage is very low. When the output voltage is close to 18V, the IR11682 will start to operate.
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